Fluctuations in the concentration of calcium ions (Ca ions) may play an important role in regulating the assembly and function of the macromolecules and motile processes in the mitotic apparatus (MA) of the dividing cell. It is the objective of this proposal to determine the following: 1) does free (Ca ion) change during mitosis? 2) does experimental alteration of free (Ca ions) promote or inhibit mitosis?, 3) where is calcium localized in the dividing cell? The metallochromic dyes, murexide and arsenazo III, will be used in conjunction with two wavelength dual beam spectrophotometry to determine changes in free (Ca ions) in synchronously dividing populations of cells. The photoprotein aequorin will be used in endosperm tissue of Haemanthus and Tilia, and coenocytes of Blastophysa to determine the timing and possibly position of (Ca ions) increases in single dividing cells. In the second part of the proposal chelators (EGTA and EDTA), and ionophores (A23187 and X537A) will be used to modulate intracellular levels of Ca ions in order to determine what conditions stimulate and/or inhibit mitosis. Finally, bound calcium will be localized in the dividing cell by electron microscopic cytochemistry using a) the pyroantimonate-osmium reaction, b) calcium oxalate precipitation and c) high (Ca ions) fixation.